# Re: \[PATCH v2\] x86/paravirt: Don't make vcpu\_is\_preempted() a callee-save function

- From: Peter Zijlstra \<peterz@infradead.org\>
- Date: 2017-02-13 10:54:09 
- List: virtualization
- Page: https://ratatoskr.run/virtualization/2017/02/4933119
- Thread: https://ratatoskr.run/linux-arch/2017/02/4932016/t.md

```text
On Mon, Feb 13, 2017 at 11:47:16AM +0100, Peter Zijlstra wrote:
> That way we'd end up with something like:
> 
> asm("
> push %rdi;
> movslq %edi, %rdi;
> movq __per_cpu_offset(,%rdi,8), %rax;
> cmpb $0, %[offset](%rax);
> setne %al;
> pop %rdi;
> " : : [offset] "i" (((unsigned long)&steal_time) + offsetof(struct steal_time, preempted)));
> 
> And if we could get rid of the sign extend on edi we could avoid all the
> push-pop nonsense, but I'm not sure I see how to do that (then again,
> this asm foo isn't my strongest point).

Maybe:

movsql %edi, %rax;
movq __per_cpu_offset(,%rax,8), %rax;
cmpb $0, %[offset](%rax);
setne %al;

?
```